The passband of accelerometers used in the strong earthquake motion networks usually starts from zero. Its sensitivity can be considered the same and the linearity characteristic is consistent among the various frequencies points during the whole passband. Consequently, characteristics of the seismometers in the passband (especially in the low frequency passband) can be acquired according to testing and calibrating the sensitivity and linearity precisely under zero frequency (or ‘static state’).
At present, the electrically powered tilting platform made for optical calibration is a kind of precision tilting platform typically used, in which stepper motors and worm drives are utilized. As the existence of transmission clearances owing to worm drives, the positioning precision is limited. Furthermore, the transmission efficiency is low because of the friction loss of the worm drives. Though semi-closed loop control is realized when the optional encoder is mounted, full-closed loop control is not realized. Consequently, the positioning accuracy is affected. Moreover, the tilting angle is figured out by the displacement of the stepper motor and no precision device is used for measuring the tilting angle directly. Thus, measuring error occurred.